The present invention relates to a harvesting machine or field chopper comprising an attachment, an input feed device, a cutting drum, an ejection device and associated drives whereby the input feed device is driven hydrostatically and the cutting drum is driven mechanically. Foreign body detection devices and conditioning devices can also be provided in the path of harvested or chopped up materials. This type of machine is used in farming and is either self-propelled or a pulled by a towing vehicle.
The feed device in harvesting machines or harvesting machines of this type is drive in a known way by chains or by propeller shafts. The cutting length of the harvested materials is adjusted by changing the gear or transmission ratio in the drive train of the feed device in order to change the cutting length in a desirable manner by changing the feed speed while maintaining a constant chopper drum rotation speed. German Patent Application DE-OS 28 19 200 discloses a hydrostatic drive for the feed device of a harvesting machine instead of a mechanical drive unit. Because of this hydrostatic drive the harvesting machine has the additional advantages of reduced structural width, reduced wear of the drive components, a continuous control of the cut or chop length and an easy reversibility For example, German Patent DE 35 05 887 discloses that methods are known for controlling the forward speed of the harvesting machine according to the machine load. In this patent however measures or features are not taken based on a consideration that the cutting length of the harvested materials is increased because of overloading by an excessive forward speed with a measured rotation speed drop of the cutting drum, which has a negative effect on the quality of the harvested crops or materials. In the disclosed system the cutting proceeds with an increased cutting length until the speed is adjusted so that the chopper drum again reaches its set or desired rotation speed. Considerable differences in the yield per unit area of the harvested materials occurring because of differences in the ground fertility, ground water content, shade provided by trees in and beside the field, fertilization differences, weed density or soil density can require a continuous regulation of the forward speed of the harvesting machine during its forward motion, in which the described variations of the cutting length occur during the control process. However without differences in the harvested quantities a one-time preset cutting length cannot always be maintained in a hydraulic system, because the power of the hydraulic drive changes because of different oil leakage amounts caused by pressure differences. The cutting length of the harvested materials can be maintained a constant only by changing the forward speed of the harvesting machine.